Charging a battery with a battery?

Wurkkos

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Ok, not quite about flashlights, but...

I happen to have a bunch of 6v, 4.5ah lead calcium SLA batteries with not much to do. (the type used for backup in exit signs and such) As I understand it, these are the "LSD" version of lead acid batteries. I was thinking of making a six pack (18v, 9ah) of batteries to keep in my truck for emergency jumpstarting. I don't think the batteries are strong enough for a direct "jump", but if I made an 18v pack, and attached that to a "dead" 12v battery, would it charge? Would I need a resistor or something between the batteries to stop an infinate amount of current from flowing? Or would it be smarter to try the same thing with a full 12v battery pack and dead 12v battery?
 

123Sven321

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I think a battery-pack with 12V will do fine.
When your starter battery is really dead it has less than 12V (maybe 10 or 11)
In this case the charged 12V backup (which has about 13,5 or 13,8V) will charge the dead starter battery.

When your backup battery pack had 18V you would discharge it until every cell has 4V (starting from about 6,5 to 7V). As far as I know this extremely hard and deep discharge will kill the lead batteries quickly.

Sven
 

BatteryCharger

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When your backup battery pack had 18V you would discharge it until every cell has 4V (starting from about 6,5 to 7V). As far as I know this extremely hard and deep discharge will kill the lead batteries quickly.
I should mention these lead-calcium batteries are meant for long term backup, but I don't think they last very many cycles even in the best circumstances. I'm not totally sure though about lead calcium batteries...

Otherwise they'll probably just sit in my garage gathering dust until they get too old to be usefull anymore. :shakehead
 

Lynx_Arc

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I wouldn't try and use 18v of SLA to try and charge a car, if the battery is damaged and doesn't absorb all the energy dropping the 18v it could leach into your electrical system and fry something as I don't think things are designed to take that high of a voltage perhaps 15-16v max and 14-14.5v is about the top of what alternators are supposed to charge at if I remember correctly. anything higher than 14v could possibly damage something.
 

SilverFox

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Hello BatteryCharger,

If you have at least 10' jumper cables, and don't leave the jump pack hooked up for more than about 5 minutes, and don't have surge voltage sensitive equipment on the vehicle you are jumping... It should work. As long as the battery you are jumping is in excellent condition.

If the battery is not in good condition, it can explode.

Your 18 volt jumper pack, when fully charged, will have a peak voltage of around 22 volts. A dead vehicle battery has a voltage of around 10.5 volts. When you hook those two batteries together, there will be a surge and the voltage will jump up. If you left them hooked up for an extended period of time (like longer than 5 minutes), both packs would equalize at around 16 volts. This is a little over the high edge of voltage for a 12 volt system, so voltage sensitive equipment may blow.

I think it would be much better to just hook 2 batteries together and go with a 12 volt 4.5 Ah jumper pack.

Tom
 

BatteryCharger

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If you left them hooked up for an extended period of time (like longer than 5 minutes), both packs would equalize at around 16 volts. This is a little over the high edge of voltage for a 12 volt system, so voltage sensitive equipment may blow.

Yeah...I was kind of counting on the SLA batteries to be totally dead by the time everything equalized, so as not to over-volt the battery...but that wouldn't be much more than a guess.

That said, my old boat likes to charge it's starting battery up to 17 volts. It's a 9 year old cheap store brand battery, and I've never even had to add water. :thinking: The electronics on my boat aren't very sensitive though...

Maybe I'll look for some kind of DC-DC converter to "pump" power from whatever voltage my backup pack is, into the car battery at closer to 14v.

(I have more than one of those large portable jump packs, but I'm trying to make something a little smaller/lighter)
 

SilverFox

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Hello BatteryCharger,

I don't think that will work.

Lead acid batteries follow a CC/CV charge algorithm. With your 18 volt battery, the regulator would quickly see its maximum 22 volts and cut the current back to nearly nothing. Given enough time, and sun, you may get a full charge, but I don't think it will work.

Tom
 

fivemega

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Ok, not quite about flashlights, but...

I happen to have a bunch of 6v, 4.5ah lead calcium SLA batteries with not much to do. (the type used for backup in exit signs and such) As I understand it, these are the "LSD" version of lead acid batteries. I was thinking of making a six pack (18v, 9ah) of batteries to keep in my truck for emergency jumpstarting. I don't think the batteries are strong enough for a direct "jump", but if I made an 18v pack, and attached that to a "dead" 12v battery, would it charge? Would I need a resistor or something between the batteries to stop an infinate amount of current from flowing? Or would it be smarter to try the same thing with a full 12v battery pack and dead 12v battery?
This is not a good idea at all because when they first connected together, extreme high current rush may exceed their maximum safe current and will kill 18 volt battery pack.
Also bad for your truck electronic system even when switch is not on.
Remember that computer memory system and clock take small amount of current when switch and ignition are off.
Also dome light or under hood light might go on when opened and blow by high voltage.
Many years ago I made a 7 cell (14 volt, 60Ah) battery out of two car battery for jump starting purpose.
High current rush of first connection to dead battery was about 25 Amps.
My recommendation is to buy a good and new battery and forget about jump start.
If you have some high drain accessories when engine is off, easiest solution is large solar system.
Last thing I may consider is an auxillary battery system.



Your 18 volt jumper pack, when fully charged, will have a peak voltage of around 22 volts. A dead vehicle battery has a voltage of around 10.5 volts. When you hook those two batteries together, there will be a surge and the voltage will jump up. If you left them hooked up for an extended period of time (like longer than 5 minutes), both packs would equalize at around 16 volts.
Tom,
This might be almost true if weak 12 V battery and fully recharged 18V battery both have same capacity.
It is obvious that an 18V, 4 AM battery can not equalize a depleted 12V, 80Ah battery to 16 volt.
 

BatteryCharger

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My recommendation is to buy a good and new battery and forget about jump start.
:crackup:

You don't realize I have a fleet of 9 vehicles to maintain spread out over three locations. :grin2: The newest one being a 1996 model year...

No matter how good the battery is there's always the possibility of leaving the lights on accidently, listening to the radio too long, whatever. In my daily driver I have two marine starting/deep cycle batteries and a 200 amp alternator. :)

As a side note, most of the "jumper packs" have about a 17-20ah 12v SLA - I have no trouble cranking a big old 454 truck engine with one of those.
 

cityevader

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:crackup:

You don't realize I have a fleet of 9 vehicles to maintain spread out over three locations. :grin2: The newest one being a 1996 model year...

No matter how good the battery is there's always the possibility of leaving the lights on accidently, listening to the radio too long, whatever. In my daily driver I have two marine starting/deep cycle batteries and a 200 amp alternator. :)

As a side note, most of the "jumper packs" have about a 17-20ah 12v SLA - I have no trouble cranking a big old 454 truck engine with one of those.


I've been a Ford mechanic for quite awhile, and have seen overcharging alternaters only three times. Two of which were approx 18v in late '90's trucks, zero negative effects on electronics. The last one was 26v on early 2000's truck, and I had to replace several fried electronic modules.

Personally, I wouldn't have any qualms with connecting 18v to the vehicle in terms of electronics.

However, I would worry about the spark igniting battery gases when first connecting to dead battery, as I've also seen 4 exploded batteries in my career. Never experienced it happening, only the aftermath.

Also, it wouldn't stay 18v (or22v nominal) for long after connecting because the load of the dead battery would be so high it would pull the voltage down rather quickly i'd imagine.
 
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